Space based remote sensing is an accepted means used to study various topics of scientific interest including global warming and climate change studies, and conditions on and around planets, natural satellites, asteroids, comets, and other astronomical objects of interest. Radiometric calibration maximizes the usefulness of data collected via space based radiometry.
Many space based sensors that work in the visible spectrum utilize diffusing plates that use solar irradiance as their primary on-board source of radiometric calibration. However, this is not always possible due to the lack of clear lines of sight to the sun, stray light, or other reasons. Spacecraft on an interplanetary trajectory also cannot use the standard techniques because the local solar irradiance is a function of distance from the sun, and that distance is constantly changing. Further, required access to sunlight for calibration purposes may not be available due to spacecraft structure, other payloads, antennas, or fixed attitude constraints. Rapidly changing angles of incidence may greatly complicate the use of the sun as a source of radiometric calibration.
Spaced based sensors that work in the infrared (IR) spectrum usually employ on-board blackbody simulators that produce calibrated radiances. The spectral radiance produced by an on-board blackbody is a function of the temperature of the blackbody. If higher or lower spectral radiances are required, the temperature of the blackbody is raised or lowered. This change in temperature also changes the spectral shape or color temperature of the radiance emitted, which may be undesirable for sensors that have passbands in a steeply sloped region of the spectrum.
An integrating sphere allows the production of different radiance levels without also changing the color temperature of the emitted radiance. An example of an integrating sphere used in a space borne application is found in Franklin et al., U.S. Pat. No. 5,302,823. Other references relating to integrating spheres include Snail et al., U.S. Pat. No. 6,927,850, and an article entitled “Spectral Radiance of a Large-Area Integrating Source,” James H. Walker et al., Journal of Research of the National Institute of Standards and Technology, Vol. 100, No. 1, January-February 1995, pp. 37-41. Walker's article discloses larger sized integrating spheres provide radiometric measurements of higher accuracy.
Of concern in performing calibration measurements with an integrating sphere in constrained environments is the volume taken up by the integrating sphere.